The complexity of internal combustion engine control systems has increased over the years in an attempt to draw an optimum compromise between engine performance, efficiency, and exhaust emission content. For exmple, the correct amount of fuel metered into the engine by an electronic fuel injection system is a complex function of manifold pressure, engine speed, coolant temperature, air temperature, and throttle angle schedules. The combination of two or more of these schedules results in multiple dimension "contour maps" which are typically nonlinear and/or discontinuous. Analog techniques have heretofore been employed to provide schedules solving these contour maps. For example, as disclosed in U.S. Pat. No. 2,980,090 issued Feb. 4, 1957 to R. W. Sutton on a "Fuel Injection System," discontinuous linear segment function generator circuits and/or nonlinear function generators are utilized to approximate the contour shapes. While digital systems for solving such contour maps have only been reported heretofore, such systems store the various points of the contour map in some form of memory and employ a linear interpolator to interpolate between stored points of the function.
Current analog function generation schemes are limited in accuracy, are subject to problems of temperature drift, and ultimately are expected to be limited to a higher cost of implementation than systems utilizing digital technology. On the other hand, digital systems utilizing read-only memory appear to require a large number of words of storage in order to provide accurate function generation of the various maps. For example, a publication on one application reported that approximately 2,048 words of storage were required for a low performance system. High performance systems may require considerable more storage capacity. The present invention recognizes that certain three dimensional internal combustion engine schedules and particularly schedules for moderate performance machines are closely approximated by as few as just two component schedules, one of which comprises a fixed logarithmic segment.
It is well known that the ignition spark advance angle must be increased as a function of engine speed in order to allow for the slow combustion of the fuel. It is also well known that good acceleration characteristics require that the ignition spark advance angle be decreased for acceleration.
Past ignition advance schemes have consisted mainly of providing speed advance by the use of mechanical centrifical advance mechanism on the distributor. Retard of the advance mechanism is provided by a vacuum advance control on the distributor. It is believed, however, that these mechanisms will not provide sufficiently accurate spark advance control in order to allow meeting Federal emission standards while yet providing a vehicle with good driveability.
Matrix studies have been conducted on automobile engines which indicate that further, more complex control of the spark advance angle can become an important factor in reducing the air pollution generated by an internal combustion engine utilizing the otto cycle. Such studies have been reported in "A Study on Spark Ignition Control Variables" by T. W. Keranen and H. P. Wertheimer, Bendix Technical Journal, Vol. 4, No. 3, 1971. Data published in the Wertheimer reference, supra, indicates that the control of the ignition spark advance will consist primarily of the solution of a three dimensional schedule involving two sensed engine parameters, such as absolute manifold pressure and engine speed.
Accordingly, a scheme of ignition spark advance control is disclosed here which will allow precise control of the engine firing angle by the use of an electronic digital computer. The advantage of this approach to the traditional mechanical and vacuum advance systems is that the specific contour of control schedule required of the engine may be generated more accurately by the electronic computer. An additional cost advantage is expected to occur in the form of time shared electronics which may be utilized, in a preferred implementation, to control both fuel injection and spark advance.